1. What will happen to alt coins when Bitcoin becomes a legal tender? Will they have a massive growth,or would people sell and jump into Bitcoin?
Probably same thing as in the past. Some (even top marketcap) coins will die before that, but naturally some of them get pumped.
2. What would happen to alt coins when Bitcoin becomes relatively stable( when it is fully mined and will solve its problem of price volatility)? Will alt coins survive the test of time? Will new alt coins be created later? What would become of the various countries'currencies, especially the Dollar (the global reserve currency) after Bitcoin's general acceptance?
Actually this will most likely be the golden age of altcoins. People will most likely use btc as a pair for trading more then fiat money, as there are benefits for holding permissionless and not volatile coin.
3. If alt coins survive in the next century what is the reason it would.
Does it need a new reason? Reason would be that they been around so far. Even though some of them will die, very different solutions to bitcoin will most likely be continued to be explored.
4. Should alt coins be traded rather than HODL?
If you can successfully trade them then by all means do that. But hodling shouldn't mean never selling. You need to have a plan what are you going to do with it when you have enough wealth.